a) Field of the Invention
The present invention relates to a linear image sensor chip and a linear image sensor, and more particularly to a linear image sensor chip and a linear image sensor for reading an image.
In this specification, a linear sensor for reading an image is called a “linear image sensor”, and a semiconductor substrate having an image pickup section for a linear image sensing is called a “linear image sensor chip”.
b) Description of the Related Art
A linear image sensor used for various apparatus such as facsimiles, electronic copiers, image scanners and bar code readers is constituted of a linear image sensor chip (hereinafter abbreviated to “LIS chip”) and a package accommodating the LIS chip.
An LIS chip has an elongated semiconductor substrate, an image pickup section formed on the semiconductor substrate, peripheral circuit sections, and a plurality of bonding pads.
The image pickup section includes at least one photodiode group and a charge transfer element provided for each photodiode group. Most LIS chips have horizontal or vertical overflow drains for draining electric charges in photodiodes. A horizontal overflow drain is formed in the image pickup section.
The number of photodiode groups to be formed in the image pickup section is changed with application, performance and the like of the linear image sensor. An LIS chip used for a linear image sensor for taking a black and white image has generally one or two photodiode groups, whereas an LIS chip used for a linear image sensor for taking a color image has, for example, three photodiode groups.
Each photodiode group is composed of a plurality of photodiodes formed in one surface of an elongated semiconductor substrate and disposed on one line along a longitudinal direction of the semiconductor substrate. The number of photodiodes constituting one photodiode group changes with application, performance and the like of the linear image sensor.
For example, some LIS chip used for an image scanner has only about 256 photodiodes. In an LIS chip used for a facsimile, an electronic copier and the like, each photodiode group has about 2,000 or more photodiodes and some group have more than 10,000 photodiodes.
The charge transfer element is made of, for example, a CCD (charge coupled device). A charge transfer element made of a CCD includes a charge transfer channel formed in the semiconductor substrate and a number of transfer electrodes formed on an electrically insulating layer on the charge transfer channel.
In order to control transfer of electric charges from a photodiode to a charge transfer element, a readout gate is formed for each photodiode. For example, the readout gate is constituted of a readout gate channel formed in the semiconductor substrate and a readout gate electrode formed on an electrically insulating layer on the readout gate channel. The readout gate electrode may be formed separately from a transfer electrode constituting the charge transfer element, or may be formed as a portion of the transfer electrode constituting the charge transfer element.
The peripheral circuit includes at least one output amplifier electrically connected to an output terminal of the charge transfer element formed in the image pickup section.
The bonding pads are formed along the peripheral area of the semiconductor substrate either in the central area of the LIS chip or in the opposite end areas along the longitudinal direction. The number of bonding pads is approximately 16 to 48 excepting those used for product test.
Each of bonding pads for external connection has an exposed surface. The bonding pads used only for product test have in some cases a monochromatic light-suppressing layer formed thereon.
A package for accommodating an LIS chip has a bottom portion, sidewall portions and a lid portion. The bottom and sidewall portions are made of light shielding material. The lid portion has a window made of transparent material.
The lid portion with the window may be constituted of a glass lid and light shielding material covering the peripheral area of the glass lid.
A plurality of lead electrodes passes through the sidewall portions from the inner space to outer space of the package.
Each lead electrode is electrically connected to a corresponding bonding pad by a bonding wire.
A dynamic range of a general image reader utilizing a linear image sensor is about 40 to 50 dB. A dynamic range of a high-grade image reader utilizing a linear image sensor, e.g., an image reader used for forming a print original, is about 60 to 70 dB or more.
Because of a high dynamic range of the high-grade image reader utilizing a linear image sensor, even small noises, which cannot be detected by the general image reader, are detected.
Noises generated in the linear image sensor can be reduced to some extent during processing a signal output from the linear image sensor to obtain an image signal. However, even in this case, a conventional high-grade image reader sometimes has local ghost-like noises on a reproduced image or picture.
An image reader having a broader dynamic range is desired nowadays. It has been desired to further suppress generation of noises in a LIS chip or a linear image sensor.